The present invention relates to rotary fluid pressure devices of the type including a gerotor displacement mechanism, and more particularly, to those of the "sealed star" type.
Although the present invention may be used advantageously in a gerotor motor or a gerotor pump, it is especially suited for use in a fluid controller such as the steering control unit (SCU) of a full fluid-linked hydrostatic power steering system, and the invention will be described in connection therewith.
Those skilled in the SCU art have, for many years, been attempting to reduce "wheel slip", i.e., a condition whereby, when the steering cylinder reaches the end of its travel, the steering wheel is still able to be rotated by the vehicle operator, as a result of fluid leakage within the SCU. Typically, the leakage is occurring between the rearward surface of the gerotor star and an adjacent surface of an endcap member.
One conventional way of dealing with the problem of wheel slip is to reduce the gerotor side clearance and increase the torque on the bolts which fasten the gerotor gear set and the end cap to the main housing of the SCU. However, in certain SCU applications, increased bolt torque is undesirable because it can cause binding of the gerotor star, and it is necessary to reduce wheel slip in some other manner. Binding of the gerotor star is undesirable because it interferes with the precise metering characteristics of the SCU, and effects manual steering and the follow-up capability.
U.S. Pat. No. 4,145,167 illustrates one approach utilized by those skilled in the SCU art, the approach being referred to as a "sealed star" in which a sealing arrangement is disposed on the rearward surface of the gerotor star, in sealing engagement with the adjacent surface of the endcap. In the conventional, prior art, sealed star arrangements, the intention is to prevent leakage of fluid through the gerotor side clearance to the case drain region of the SCU, which is connected to the system reservoir. In this prior art arrangement, the sealing is accomplished by means of an axial squeeze of the seal assembly, i.e., by compressing the seal assembly axially between the bottom surface of the seal groove and the adjacent surface of the endcap.
In many SCU applications, the conventional "axial squeeze" type of sealed star arrangement has been generally satisfactory. However, one inherent disadvantage of the prior art sealed star was that the amount of axial squeeze on the seal assembly was critical, and had to be accurately controlled. Obviously, insufficient squeeze on the seal assembly would likely result in leakage, thus permitting wheel slip. On the other hand, excessive squeeze on the seal assembly would require excessive input torque in order to manually rotate the gerotor star (as is required for manual steering).